Signal coupling network



Feb. 6, 1940. o. E, FOSTER SIGNAL COUPLING NETWORK Filed Nov. 17, 1937 2 Sheets-Sheet 1 FREQUENCY 70 NEXT STAGE 70 usxrsmes INVENTOR. v /DZDLEY E. FOSTER BY m ATTORNEY.

Feb. 6, 1940. D. E. FOSTER 2,189,063

SIGNAL COUPLING NETWQRK Filed Nov. 17, 1937- 2 Sheets-Sheet 2 4 7'0 SIGNAL 70 NEXT STAGE SOURCE i C2 Rf 70 NEXT STAGE INVEN TOR.

71,5025) E. F057ER BY ATTORNEY.

Patented Feb. 6, 1940 UNITED STAT SIGNAL COUPLING NETWORK Dudley E. FosterfSouth Orange," N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 17,1937} seria1 No. 174,928 islclaims. (01. 2504.0)

It is often desired in radio receivers, such as those employed in the broadcast band, to utilize between the, antenna circuit and the first tunable radio frequency amplifier circuit, a coupling network which has a high, substantially uniform gain characteristic without undue effect on the normal tuning of the selector circuits. Again, such a characteristic is desired for interstage coupling networks. Particularly in the case of automobile radio receivers, where the signal collector is physically small, it is of great importance to employ, between the collector and the tunable input circuit of the first amplifier, anetwork which has acharacteristic of the type described above, ,In general, it may be stated I that it is desirable to employ for a high frequency coupling network, circuit elements which permit of obtaining, at the will of the designer, higher gain at the higher frequencies,v or lower'frequencies, of the tuning range; or substantially uniform gain throughout the receiver tuning range. V p

Accordingly, it may be stated that it is one of the main objects of .my present invention to provide a high frequency couplingfnetwork between two high 'frequencyfcircuits, and one of which circuits is a tunable circuit, the'coupling network including a capacitive reactance common to the low alternating potential ends of the I coupled circuits to provide a coupling which varies inversely with frequency; whereas coupling is provided between the high alternating potential ends of the coupled circuits by a capacitive reactance which furnishes a coupling varying directly with frequency.

Another important object of myinvention is to provide a coupling network between a source of radio frequency energy and. a tunable radio frequency circuit, and'which coupling: network consists of a pair of capacities connected between-the low and high alternating potential ends'of the coupled circuits; and the twocapacities furnishing a coupling characteristic which may be'made to vary in a predetermined manner with respect to frequency.

. the collector and selector circuit; the signal en- Another object of my present invention is to provide a coupling .network between a signal collector andra 'tunable'signal selector circuit;

'of the inherent antenna capacity.

ergy. transfer through one of these capacities varyingdirectly with frequency, and the signal energy transfer through the otherof said ca,- pacit-ies varying inversely with frequency.

Still other objects of my invention are to improve generally the simplicity and efficiency of high frequency coupling networks, and more especially to provide signal coupling networks for radio receivers which are not only reliable and efiicient. in operation, but are readily manu-I factured and assembled in radio receivers.

The novel features which I believe to be characteristic of my invention. are, set forth with particularity in the appended claims; the inven-' tion itself, however,- as to both its organization ES PATENT" O IC and method of operation, will best be understood by reference to the following description taken inconnection withthe drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be car ried into elfect.

In the, drawings: 7

Fig. 1 shows a circuitdiagram of a generalized coupling network embodying the present invention; v

i Fig. Zgraphically illustrates the characteristics of the coupling network;

Figs. 3, 4, 5 and 6 show respectively different embodiments of the invention; and Fig. 7-shows a modified type of signal coupling network.

Referring now to the accompanying drawings,

wherein likereference characters in the difierent figures designate similar circuit elements; there is shown .in Fig. 1 a coupling network adapted to be employed between a source of signal energy and a utilization device; For example, the source of energy may be the signal collector-ofan automobile radio receiver, while the utilization device can be the first-signal amplifier tube of the receiver. The coupling network comprises a primarycoil L1 and a secondary coil L2. I There is no magnetic coupling between. these coils, and the. junction of the coils. is, connected to thelow alternatingpotential side" of the network through a condenser C 1.

The high-alternating potential ends of the two.

coils are connected by a second condenser C3. Aconclenser C4 is arranged in series with coil L1, and the variable tuning condenser C2 is connected in shunt with the series path including coil L and condenser C1. When thesou ce of. voltage e1 is an antenna, condenser. C consists of voltage e2.

coupling elements between the primary coil L1 and the tunable circuit L2-C2. The impedance of condenser C1 varies inversely with the frequency of the signal energy impressed across coil L1. I Hence, for a given current in the circuit C4L1-C1, the voltage across condenser C1 will likewise vary inversely with the frequency of the impressed signal energy. Fora given .voltage across condenser C1, the voltage e2, when condensers C1 and C2 in series are resonant with coil L2, is approximately equal to the'voltage across coil L2, which is proportional to @152 If now the current through con- (w=21rf). denser C1 is greatest at the lowirequency end of the tuning range of the tunable input circuit,

then the voltage e2, due to'current through condenser C1, will similarly be greatest at the low frequency end of the tuning range. This is illustrated by the full line curve A in Fig. 2.-

In Fig. 2 there is plotted Gain against Frequency. The component of voltage e2, due to the condenser-C3, varies directly with the frequency of applied signal energy. The full line curve B in. Fig. 2 shows how the network gain, 1. e., the ratio v furnishes the gain-frequency characteristic illustrated bya dotted line curve C in Fig. 2. Condenser C3 adds to the circuit minimum capacity but in' auto" receivers, where high gainis desired, the range extends only to 1550 kc., so that range 1 coverage is not difficult. On household receivers,

extending to 1700 or 1800 kc., antenna gain need not be as high, and in this case '03 may be made smaller, sayof the order of 5 mi.

1 For a given resonant frequency of L1- C1--C4,

' increasing the magnitude of condenser C1 will decrease the signal energy transfer through C1. Since this latter capacity serves principally to transfer signal energy at the low frequency end of" the network tuning range, it will primarily reduce the voltage gain at that end of the tuning range. The condenser C3 functions principally to transfer signal energy at the high frequency end'of the tuning range, and an increase in the magnitude of condenser C3 will primarily increase the voltage gain at the high frequency end of the tuning range. Substantially uniform gain may be realized with very little coupling through C1 if L1 and C4 are made resonant at, or closely adjacent to, the low frequency end of the tuning range, because under such conditions'the voltage across L1 will be maximum at such frequencies and compensate for the inefficiency of C3 as a coupling means at those frequencies If circuit C4- L1-C1, be made resonant at frequencies well 1 below thelowest frequency of the tuning range, C must be made smaller in capacity to maintain coupling efficiency at the low frequency end of the tuning range.

Figs. 3 to 6 inclusive illustrate different embodiments of the fundamental coupling network depicted in Fig; 1. Hence the following description should be considered in the light of the explanation given in connection with Figs. 1 and 2...

In order clearly to relate the circuits in Figs. 3 to 6, inclusive, to the fundamental network of Fig. 1, corresponding circuit elements will be similarly designated. 1

Considering the circuit. shown in Fig. 3, there is shown a signal collector A which is coupled to the input electrodes of the first radio frequency amplifier tube I; the coupling network comprises the primary coil L1- and the secondary coil L2.

' The inherent capacity of the antenna A takes the place of C4 of Fig.1. The junction of these two coils is connected to ground through a resistor,

R1, and this resistor may have a, magnitude of the order of one-half megohm; the resistor func' tioning to complete the direct current path between the control grid of tube l and the grounded side of resistor 2 in the cathode circuit of the tube. is the customary grid bias resistor disposed in the space current'path of the amplifier, the radio It will be understood that. the resistor 2 1 frequency by-pass condenser 2* functioning in the usual manner. The variable tuning condenser C2 has its rotors at ground potential, and the junction of coils L1 and L2 is connected to ground path including the couplingthrough a second condenser C1.

The condenser C3 connects the high alternating potential ends of the primary and secondary coils. The tube l mayhave its plate circuit coupled'to the usual networks of a superheterodyne' receiver,

or a tuned radio frequency amplifier type receiver.

Furthermore, the variable tuning condenser C2 may be considered as being adjustable through the broadcast band of approximately 500 to 1500 kc. The signal'collector 'Amay'be of any desired type; V ticular value in cases such as in an automobile The present-coupling network is of parradio receiver, where the antenna must be physi- Y cally small. In such case it'is' of great import-f ance, since the collected signal'amplitude is small, to transmit the collected signalenergy to the first i amplifier tube uniformly andwith high gain.- The network shown in Fig. 3 will function to pro duce such a high, and uniform, gain-frequency characteristic. "In Fig. 3, L1 resonates the com-v bination of C1; and; the antenna capacity at, I or near, the low'frequencyiend of the tuning; Wherever the condenser C4 is used. the

range. v latter together with C1 resonates L1. at,or near,

' the low frequency endof the tuning range.

In Fig." 4, the radio frequency amplifier tubes 3' and 4 are shown coupled by the coupling'net-- work-of my invention. The tube 3 has its input electrodes coupled across'fthe-tunable input cirf cuit 5; the latter may be coupled toflanysignal source, such as a signal'collector device," or'even" a prior tunable amplifier stage. -q'.l."he grid bias networkt is'disposed in the g'rounded cathode' lead of tube3; the plate of tube -3is connected' to apositive potential source through coil L1.

The secondary'coil L2 has its high potentialend connected tofthe" plate' end of 'primajrycoil L1 through a condenser C3. The condenser C1 conmeets the low-potential end Of'COiIJLo to'ground;

the condenser C4 connects-theplate end' of coil L1 to the junction of coil L2 and condenser C1 The condenser C5 acts as a resonating condenser for the coil L1. The capacityof condensers C4-C1-C5, the'leifect of Ca being negligible rjesonateL1at, or near, the low frequency-end of the tuning range. The portion of the low free quency resonated circuitwhich comprises C1 is,

in series.

connected to ground through condensers C4 and also, part of the tunablecircuit, and acts as a coupling to the latter.

The variable condenser C2 is shunted across coil L2 and condenser C1; the resistor R acts as the direct current path for the grid of tube A, and it connects the low potential end-of coil L to the grounded terminal of grid bias resistor l. The condenser C'acts as a carrier by-pass condenser for the low potential end of coil L1. Asexplained previously,,the condensers C3 and C1 function to providethe gain-frequency characteristics which will result in the curve C of Fig. 2. Variable condensers and C2 will be uni-controlled, so as to tune circuits 5 and L z- C2 to the same carrier frequencies of the tuning range.

In Figs, 5 and .6- are shown'modifications of the coupling network in Fig. 4-. Thus, in Fig. 5

the primary coil L1, condenser'Cx and condenser cuit, has its high potential end coupled to the high potential end of coil L1 through condenser C2; the low potential end of coil L2 is connected to the junction of condensers C4 and C1. The remaining elements of the network are the same as in Fig.. i. i

In Fig. 6 the coupling network is varied in construction by feeding the plate current to tube 3 through the primary coil L1 and choke coil 10 The junction of these two coils is C1 in series. The radio frequency bypass condenser C5 is connected between the junction of coil L2 and resistor R1. and the junction of condensers C4 and C1. In both Figs. 5 and 6 the condenser C3 functions, to transfer signal energy Coil L1=370 microhenries Coil L =O microhenries Condenser 'C1=2900 mmf. Condenser C3=15 mmf.

tenna A is connected to the junction of coils 22 and '23 through condenser 2|. The cathode of tube 21 is connected to ground through a path including coil 23 and self-biasing resistor 24, the latter being bypassed for radio frequencies by condenser 25. The variable condenser 26 tunes the coil 22 over the desired range of carrier frequencies.

It can be shown that at a frequency to be suppressed if 1 v inn L23 C26 v then no voltage at such frequency is developed between the grid and cathode of tube 21. In other words, with such relation, no voltage at the frequency to be suppressedv is developed across" l. A coupling network for transmitting energy with substantially uniform gain from a source of signal energy to a resonant input circuit, said resonant circuit including a coil and means for varying its frequency over a wide frequency range, said network including a second coil connected to said source, condensivemeans coupling the high potential ends of the two coils, and an additional condensive means coupling the low potential ends of said coils.

2. A coupling network as'defined in claim 1, said second coil being untuned, and the signal transfer through said ;two condensive means being in opposed relation.

3. A coupling network. for transmitting energy with substantially uniform gain from an antenna circuit to a resonant input circuit connected to a discharge tube, said coupling network comprising a primary coil connected in said antenna circuit, said; resonant circuit including means for adjusting the tuning thereof over a desired signal frequency range, and a pair of independent capacity coupling paths between said primary coil and said resonant input circuit for transferring signal energy between the antenna circuit and the tube in opposed relation.

4. A coupling network, adapted to transfer signal energy in a desired signal frequency range,

comprising an untuned primary coil upon which is impressed signal ener y, a secondary coil, means for tuning said secondary coil over said range, a condenser, coupling the high potential ends of said two coils, transferring signal energy most efiiciently at the high frequency end of said tuning range. and a second condenser, connected between the low potential ends of said two coils, transferring signal energy most efficiently at the low frequency end of said tuning range.

5. In a network as defined in claim 4, there be ing zero magnetic coupling between said two coils.

6. A coupling network arranged to connect a pair of amplifier tubes in cascade, said coupling network comprising a primary coil and a secondary coil; there being zero magnetic coupling be" tween said coils, means for resonating solely said secondary coilto a desired signal frequency, and a'pair of independent condensers coupling the high potential ends of said coils and the low potential ends of said coils.

'7. In a network as defined in claim 6, an additional condenser connected in series with said primary coil for resonating the latter.

8. In anetwork as defined in claim 4, a third condenser connected in series relation with said primary coil and said second condenser.

9. Ina network as defined in claim 6, said high potential coupling condenser transferring signal energym'ost efiiciently at the high frequency end of the tuning range.

" 10. A coupling network, adapted to transfer signal energy in a desired frequency range, comprising an untuned coil and. a tuned coil with zero magnetic coupling between them, the coupling between the circuits connected to the two coils comprising two condensers, one connecting the high potential ends of the coils, and the other being at the low potential ends of the coils and common to the circuits of both coils.

11. A signal coupling network, adapted to transfer energy in a desired. frequency range, comprising an untuned coil and a tuned coil arranged to have zero magnetic coupling between them, a condenser connected to the high potential terminals of said coils whereby said condenser acts as the predominant coupling means at one end of the frequency range, and a second condenser connected. to the low potential terminals of both coils and serially connected there with whereby said condenser acts as the predominant coupling means at the opposite end of the frequency range.

12. A coupling network, as described in claim 10, wherein one of said coils is tuned to a frequency adjacent to the low frequency end of the desired range, and the other of the said coils includes in the circuit connected thereto means for tuning over the desired frequency range.

13. A coupling network, as defined in claim 10, wherein one of the coils is tuned to a fixed frequency within the desired range, andthe other of said coils is connected in a circuit including means for tuning that circuit over the desired frequency range.

14. A signal coupling network, adapted to transfer energyin a desired frequency range, comprising a tunable circuit consisting of a pair of series connecting coils and a variable condenser, a third coil connected between the high potential terminal of said series connected coil pair and the high potential input terminal, a fixed condenser connected between the high potential input terminal and the junction between the aforementioned series connected pair of coils,

the output from the network being taken across the higher potential coil of the series pair, said third coil and said fixed condenser being so proportioned that when the network is tuned to transmit a desired frequency, the transmission through the network of an undesired frequency v the bridge network, only one of said condensers being variable, the signal input being impressed across one diagonal of the bridge, and thesignal output being taken across a third coil which is untuned and connected across the other diagonal of the bridge network. a

16. A signal coupling network consisting of a fixed condenser, one coil, a variable condenser and a second coil connected in a closed mesh thereby forming the arms of a bridge type network, an untuned third coil connected between the junction of the variable condenser and one coil and the junction between the fixed condenser and the other coil, the input being across the two junctions to which the thirdcoil is not connected, the coils and the fixed condenser forming the bridge arms being proportioned to suppress an undesired frequency, so that when" the variable condenser is adjusted to obtain max-:

imum output of a desired frequency across the said third coil, the bridge is balanced for the .un-

desired frequency and substantially nooutput of the undesired frequency is obtained.

DUDLEY E. FOSTER. 

